Using an experimental matching-to-sample paradigm, the role of crossmodal and intramodal stimulus equivalence in elementary reading comprehension is being studied. Basic studies of stimulus-class formation are being carried out with children and lower species. Also being investigated are: The role of response vs stimulus mediation in the formation of stimulus classes; the role of mediated equivalence in the generation of unpracticed language forms; problems of stimulus control and learning deficits in neurological mutants, animals of known genetic constitution and inherited CNS disease.